You can play any game as any gym leader. There's about 15-20 Pokemon for each leader to choose from. Must have at least a team of four. You may trade or hack in up to 3 Pokemon. Must use the signature Pokemon if at all possible, or it's pre-evolution (ex. Use Onix in Red if you're playing as Jasmine). The lists of Pokemon were taken from all that person's gym leader appearances, and some had to have some added (gen I and II probably have more than anyone since they were in Stadium games).

The rules are different than the other gym leader challenges, so idk what I'll do about people that are in the middle of one of the old ones that want to re-sign up. Probably just let them finish with their old rules. But all the gym leader challenges have different rules anyway, so eh. This one will be much better organized and more official, and less restrictive since you can go as Clay in Blue if you wanted to lol.